Melissa slowed down in the Caribbean on Tuesday, allowing it to build into a potentially destructive tropical storm as it moves north toward Haiti, the Dominican Republic and Jamaica, where it is expected to bring significant rain by the end of the week.
From there, there is little certainty among forecasters about where it will go or just how intense it may become.
Why?
The uncertainty has to do with a few factors, but a main one is the output of the computer weather models that help meteorologists make their forecasts. Specifically, it’s a type of map called a spaghetti plot that shows where computer simulations predict the center of a storm will be five, seven or even 14 days in the future.
These mapped model outputs get their name from their resemblance to long strands of pasta.
The closer together the lines are, the more confidence forecasters have in what a storm might do, because it means many of the models agree with one another.
On the spaghetti plot for Tropical Storm Melissa, those lines are scattered in all sorts of directions over the next few days.
Some take the storm northeast toward the Atlantic, while others show the storm stalling and drifting west back toward Central America. Many show Melissa remaining in the Caribbean Sea throughout the week and into the weekend.
One of the hardest parts of any hurricane forecast is trying to predict when a storm will turn and how much of a turn it might make. It’s also one of the most crucial, because understanding the storm’s path gives people on land enough time to prepare.
The computer models typically run four times a day. After each run, forecasters look to see if there have been changes from the last one; they look for a pattern to help build their confidence around where a storm is most likely to go.
But if there is a significant change, they usually wait for another model run before they make substantial adjustments to the guidance they turn around and share with the public.
The more data that experts have for their models, the better the forecasts are. A hurricane hunter aircraft was scheduled to investigate Melissa on Tuesday afternoon to collect more data on Melissa’s strength and structure; more trips are expected in the coming days.
Forecasters aren’t just worried about where the storm will go. They are also concerned about how intense it might become. While experts can use one model to monitor intensity, they usually prefer to look at many of them. To do this, they often plot different forecast intensities on a chart that progresses over time. This line plot gives forecasters an idea of what could happen.
Similar to the map plots, the tighter together the lines are among models, the more confident the forecasters are about the intensity. But as of Tuesday afternoon, they weren’t very confident at all, writing that there remained “significant uncertainty” in the storm’s movement and intensity.
Judson Jones is a meteorologist and reporter for The Times who forecasts and covers extreme weather.
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